1. Field of the Invention
The present invention relates to a driving method for a liquid crystal display which is suitable for a thin display, in particular, to a driving method for a liquid crystal display wherein deterioration of the image quality, such as unclearness of an outline and blurring of colors when displaying a moving image, is prevented so as to increase an image quality and a luminance.
2. Description of the Related Art
A liquid crystal display is provided with two sheets of glass substrate and a liquid crystal layer placed between them. In general, in a thin film transistor (TFT) type panel, color filters for three colors (red, green and blue) are formed on the glass substrate on the side opposite to the TFT side glass substrate where TFTs are provided. FIG. 1 is a cross section view showing a structure of a liquid crystal display according to a prior art. FIG. 2 is a block diagram showing a structure of the liquid crystal display according to the prior art.
In the liquid crystal display according to the prior art, a TFT portion 102 is provided on the surface of a glass substrate 101. The TFT portion 102 is provided with a plurality of scanning electrodes (not shown) arranged parallel to each other, a plurality of signal electrodes arranged so as to cross those scanning electrodes, and TFTs as switching elements arranged at intersections between the scanning electrodes and the signal electrodes. Accordingly, the TFTs are arranged in a matrix form. On the other hand, a polarizing plate 103 is attached to the back surface of the glass substrate 101. Here, the scanning electrodes extend in the horizontal direction while the signal electrodes extend in the vertical direction.
In addition, a glass substrate 104 is provided in parallel with the glass substrate 101, with a proper space between them, on the side of the TFT portion 102 of the glass substrate 101. A common electrode layer 105 made of a transparent conductive material and color filters 106 are provided on the surface of the glass substrate 104 opposite to the glass substrate 101. As for the color filters 106, three color types of filters, that is to say, red filters 106R, green filters 106G, and blue filters 106B, which extend in the vertical direction are provided and are arranged so as to repeat, in order, in the horizontal direction. The color filters 106 are formed by applying pigments or dyes. Here, each pitch of the filters 106R, 106G and 106B coincides with the pitch of the signal electrodes. On the other hand, a polarizing plate 107 is attached to the back surface of the glass substrate 104 to the surface opposite to the glass substrate 101.
Then, the glass substrate 101, the glass substrate 104 and the like, are integrated so as to provide a liquid crystal layer 108 by injecting a liquid crystal material between them. In this manner, an active matrix type liquid crystal display panel (LCD panel) 110 is configured.
In addition, a light source 109 and a light guide plate 111, which guides light emitted from this light source 109 so that it enters the liquid crystal display panel 110 in the vertical direction, are provided on the back surface of the glass substrate 101. A reflecting sheet, a diffusion plate (not shown) and the like are provided between the light guide plate 111 and the LCD panel 110. Then, a backlight is formed of the light source 109, the light guide plate 111 and the like. As for the light source 109, mainly a cathode ray tube fluorescent lamp is utilized.
In addition, a scanning circuit 131, which drives n scanning electrodes G1 to Gn, and a holding circuit 132, which drives (mxc3x973) signal electrodes, in total, DR1 to DRm, DG1 to DGm and DB1 to DBm are provided in the liquid crystal display panel 110. In addition, a signal processing portion 133 which processes image data and outputs the result to the scanning circuit 131 and the holding circuit 132 is provided. Moreover, a source for gradation 134 is provided which supplies voltage for the gradation display, to the holding circuit 132, associated with the output signal of the signal processing portion 133. Signals outputted from the signal processing portion 133 to the scanning circuit 131 are a clock signal and a start signal.
Next, a driving method for the liquid crystal display according to the prior art, which is configured as described above, is described. FIG. 3 is a graph diagram showing the relationship between time, which is taken along the horizontal axis, and luminance, which is taken along the vertical axis, in the liquid crystal display according to the prior art. In FIG. 3, the two-dotted broken line shows luminance set for one pixel and the solid line shows the actual luminance for the one pixel.
In the driving method according to the prior art, scanning pulses are applied in sequence from the scanning electrode G1 to the scanning electrode Gn by the scanning circuit 131 with reference to the start pulse VSP and the clock signal VCLK. Together with this, a voltage for gradation display is applied to the signal electrodes DR1 to DRm, DG1 to DGm and DB1 to DBm by the holding circuit 132.
However, since there exists a response time for the voltage applied to the liquid crystal until the full rotation is achieved, the actual luminance (solid line) cannot reach the set value immediately but, rather, rises gradually even when the voltage for gaining luminance (two-dotted broken line) is applied to a signal electrode as shown in FIG. 3. In the case of scanning at 60 Hz, time required for the scanning of one frame is approximately 16.7 milliseconds while a response time of a twisted nematic (TN) type liquid crystal is approximately 15 milliseconds.
In addition, the backlight 109 is turned on at all times. Therefore, light of the three colors of red, green and blue is emitted from the liquid crystal display panel 110 simultaneously in accordance with the extent of the rotation of the liquid crystal.
In addition, the development of an organic electro-luminescence (EL) display as a thin type display is also progressing. FIG. 4 is a schematic cross section view showing an EL element and its luminous principle and FIG. 5 is a block diagram showing the structure of an organic EL display according to a prior art.
The EL element is configured as follows. That is to say, a transparent indium tin oxide (ITO) electrode 122 is formed as a positive electrode on a transparent substrate 121 made of glass or film. In addition, on the ITO electrode 122, an organic positive hole injection layer 123 and an organic luminous layer 124 are deposited in sequence and, on top of that, a metal electrode 125 is formed as a negative electrode. Then, when a voltage is applied between the ITO electrode 122 and the metal electrode 125, light is emitted from the organic luminous layer 124 to the side of the transparent substrate 121.
The organic EL display is provided with a simple matrix system EL panel 120 where EL elements which are configured in the above manner are arranged in a matrix form. In addition, a row driving portion 135 which drives row electrodes R1 to RL, which the number is the same as that of the scanning lines, and a column driving circuit 136 which drives (mxc3x973) column electrodes, in total, CR1 to CRm, CG1 to CGm and CB1 to CBm are provided. In the column driving circuit 136, a latch circuit, which maintains the voltage based on the signal from a signal processing portion and which outputs signals for the number of column electrodes at the same time, and a constant current circuit, which converts the voltage outputted from this latch circuit into a current so as to supply it to a column electrode, are provided. In the organic EL display, a signal processing portion 137 which processes image data and which outputs the result to the row driving portion 135 and the column driving circuit 136 is further provided. Signals outputted from the signal processing portion 137 to the row driving portion 135 are the start pulse RSP and the clock signal RCLK.
Next, a driving method for the organic EL display, which is configured as described above, is described. FIG. 6 is a graph diagram showing the relationship between time, which is taken along the horizontal axis, and luminance, which is taken along the vertical axis, in the organic EL display according to the prior art. In FIG. 6, the two-dotted broken line shows luminance set for one pixel and the solid line shows the actual luminance in the one pixel.
In the driving method according to the prior art, scanning pulses are applied in sequence from the row electrode R1 to the row electrode Rn by the row driving portion 135 with reference to the start pulse RSP and the clock signal RCLK. In addition, a current for the gradation display by the latch circuit within the column driving circuit 136 is applied to the column electrodes DR1 to DRm, DG1 to DGm and DB1 to DBm by synchronizing them with the risings of the scanning pulses. In addition, a negative bias is applied so that no current flows through the non-scanning row electrodes.
Since the response speed of an EL element is sufficiently fast in comparison with that of a liquid crystal, a display with a desired luminance is instantly carried out by supplying a current into the column electrode as shown in FIG. 6.
In the liquid crystal display according to the prior art as shown in FIG. 2, however, there is a problem of unclearness of the outline and blurring of colors caused when displaying a moving image as described above. In addition, there is also the problem of a limit to increasing thinness because of the existence of a backlight. Furthermore, luminance is lowered through loss due to the color filters because light is emitted via the color filters.
On the other hand, in the organic EL display according to the prior art as shown in FIG. 5, which is an impulse type, the response speed is fast and, therefore, there is no problem of unclearness of the outline or of blurring of colors when displaying a moving image. In the case where the number of scanning lines increases together with the high definition, however, the time for a scanning pulse being applied to one row electrode is reduced accordingly and, therefore, a problem arises that the luminous duty and luminance are lowered. In addition, the width of a row electrode becomes narrower because of the increase of the number of scanning lines and, therefore, the problem arises that the patterning becomes more difficult and the yield is lowered. Moreover, since a latch circuit is required for adjusting a current amount, which is supplied to a column electrode in accordance with the image data, the configuration of the column driving circuit is complicated.
Therefore, in order to eliminate these defects, a liquid crystal display wherein an EL panel is provided in the backlight portion has been proposed (Japanese Unexamined Patent Publication No. Sho 59-97191, Japanese Unexamined Patent Publication No. Hei 11-249135, and the like).
In those publications, displays gained by combining liquid crystal display panels and EL panels are described wherein liquid crystal panels are utilized as shutters for light emitted by EL elements. In particular, in the Japanese Unexamined Patent Publication No. Sho 59-97191, a driving method is described where EL elements are made to emit light after the transmittance in the liquid crystal is saturated. That is to say, the timing according to which a driving signal is supplied to the EL panel is delayed in comparison with the timing according to which a driving signal is supplied to the liquid crystal display panel. In this driving method, the emitting of light of three colors for forming one pixel is carried out by time-sharing. That is to say, light of three colors is emitted according to different timings.
However, this is intended to solve the problems in a liquid crystal display and, therefore, though problems particular to a liquid crystal display can be solved by simply combining a liquid crystal display panel and an EL panel, the problems particular to an organic EL display cannot be solved. For example, the problems such as a decrease of contrast and luminous duty, together with high definition as well as difficulty in patterning still remain. In addition, though in the case where the driving method described in the Japanese Unexamined Patent Publication No. Sho 59-97191 is applied, unclearness of the outline and the blurring of colors when displaying a moving image due to the existence of a response time in the liquid crystal can be prevented, the new problem emerges that the driving circuit and the like becomes complicated due to the necessity of a clock signal with a high frequency since light in three colors is emitted according to different timings on the EL panel.
It is an object of the present invention to provide a driving method for a liquid crystal display wherein deterioration of the image quality such as unclearness of the outline and the blurring of colors when displaying a moving image can be prevented and wherein a high luminance can be gained at the time of high definition.
According to the present invention, a driving method for a liquid crystal display which comprises an electro-luminescence portion which has electro-luminescence elements and a liquid crystal portion which has liquid crystal layer, scanning electrodes and signal electrodes and controls the transmittance of light emitted by the electro-luminescence elements, comprises the steps of applying scanning pulses in sequence to the scanning electrodes, applying a gradation signal associated with image data to the signal electrodes; and allowing the electro-luminescence portion to emit light of a plurality of colors at the same time in pixels after transmissivity of the liquid crystal layers in the pixels reaches a predetermined value. The pixels are located at the intersections between a scanning electrode to which the scanning pulse is applied and the signal electrodes to which the gradation signal is applied.
In the present invention, after the transmittance of the liquid crystal in a pixel, to which the scanning electrode that the scanning pulse is applied to is assigned, reaches a predetermined value, light of a plurality of colors is emitted in this pixel so that light of these plurality of colors is emitted at the same time and, therefore, it is possible for the frequency of the clock signal supplied to the EL panel to be reduced in comparison with the case where light of three colors is emitted by time-sharing according to a prior art. In addition, current adjustment for luminance adjustment becomes unnecessary. Therefore, it becomes possible to simplify the EL driving circuit and to secure, in a wide manner, the driving margin of the driving circuit. Moreover, since light emission time per color becomes longer, a high luminance can be gained. In addition, it is possible to prevent the unclearness of the outline, the blurring of colors and the like when displaying a moving image.
In the case where pixels included in neighboring rows are made to emit light at the same time, it becomes possible to secure a longer light emission time for one time. Therefore, even though the number of scanning lines has increased due to high definition, sufficient luminance can be gained. In addition, it becomes possible for light emitting elements to increase their longevity due to the decrease in the number of light emissions and the reduction of the load of the current.
In addition, in the case where electro-luminescence portion has row electrodes which overlap the scanning electrodes, the width of the row electrodes can be made broader so that the patterning in a process becomes easier and the yield increases. At this time, it is preferable that the number of scanning electrodes that overlap each of the row electrodes are constant so that the number of rows of pixels which are made to emit light at the same time is constant. By providing such a configuration, it becomes possible to make luminance uniform in accordance with the position within the screen. The number of scanning electrodes with which each of the row electrodes overlaps may be mutually different. The row electrodes are the electrodes for applying a voltage to a unit of a row, or a plurality of rows, of the electro-luminescence elements arranged in a matrix form in the electro-luminescence portion.
In addition, the electro-luminescence portion may comprise column electrodes for each color, and, in allowing the electro-luminescence portion to emit light, all of column electrodes for the same color may be driven at the same time. In the case of adopting such a driving method, a constant current source may be provided for column electrodes of each column so that the driving circuit is simplified. The column electrodes are the electrodes for applying a voltage to a unit of one column, that is to say a unit of one color, for electro-luminescence elements arranged in a matrix form in the electro-luminescence portion.
In addition, in allowing the electro-luminescence portion to emit light, all of the column electrodes, including column electrodes for different colors, may be driven at the same time. Since it is not necessary to adjust the timing of the application of a current, the driving circuit is simplified.